Humans have bred plants and animals for thousands of years, selecting for certain commercially valuable and/or aesthetic traits. Domesticated plants differ from their wild ancestors in such traits as yield, short day length flowering, protein and/or oil content, ease of harvest, taste, disease resistance and drought resistance. Domesticated animals differ from their wild ancestors in such traits as fat and/or protein content, milk production, docility, fecundity and time to maturity. At the present time, most genes underlying the above differences are not known, nor, as importantly, are the specific changes that have evolved in these genes to provide these capabilities. Understanding the basis of these differences between domesticated plants and animals and their wild ancestors will provide useful information for maintaining and enhancing those traits. In the case of crop plants, identification of the specific genes that control desired traits will allow direct and rapid improvement in a manner not previously possible.
Although comparison of homologous genes or proteins between domesticated species and their wild ancestors may provide useful information with respect to conserved molecular sequences and functional features, this approach is of limited use in identifying genes whose sequences have changed due to human imposed selective pressures. With the advent of sophisticated algorithms and analytical methods, much more information can be teased out of DNA sequence changes with regard to which genes have been positively selected. The most powerful of these methods, “KA/KS,” involves pairwise comparisons between aligned protein-coding nucleotide sequences of the ratios ofnonsynonymous nucleotide substitutions per nonsynonymous site (KA) synonymous substitutions per synonymous site (KS)
(where nonsynonymous means substitutions that change the encoded amino acid and synonymous means substitutions that do no t change the encoded amino acid). “KA/KS-type methods” include this and similar methods.
These methods have been used to demonstrate the occurrence of Darwinian (i.e., natural) molecular-level positive selection, resulting in amino acid differences in homologous proteins. Several groups have used such methods to document that a particular protein has evolved more rapidly than the neutral substitution rate, and thus supports the existence of Darwinian molecular-level positive selection. For example, McDonald and Kreitman (1991) Nature 351:652-654, propose a statistical test of the neutral protein evolution hypothesis based on comparison of the number of amino acid replacement substitutions to synonymous substitutions in the coding region of a locus. When they apply this test to the Adh locus of three Drosophila species, they conclude that it shows instead that the locus has undergone adaptive fixation of selectively advantageous mutations and that selective fixation of adaptive mutations may be a viable alternative to the clocklike accumulation of neutral mutations as an explanation for most protein evolution. Jenkins et al. (1995) Proc. R. Soc. Lond. B 261:203-207 use the McDonald & Kreitman test to investigate whether adaptive evolution is occurring in sequences controlling transcription (non-coding sequences).
Nakashima et al. (1995) Proc. Natl. Acad. Sci USA 92:5606-5609, use the method of Miyata and Yasunaga to perform pairwise comparisons of the nucleotide sequences of ten PLA2 isozyme genes from two snake species; this method involves comparing the number of nucleotide substitutions per site for the noncoding regions including introns (KN) and the KA and KS. They conclude that the protein coding regions have been evolving at much higher rates than the noncoding regions including introns. The highly accelerated substitution rate is responsible for Darwinian molecular-level evolution of PLA2 isozyme genes to produce new physiological activities that must have provided strong selective advantage for catching prey or for defense against predators. Endo et al. (1996) Mol. Biol. Evol. 13(5):685-690 use the method of Nei and Gojobori, wherein dN is the number of nonsynonymous substitutions and dS is the number of synonymous substitutions, for the purpose of documenting natural selection on genes. Metz and Palumbi (1996) Mol. Biol. Evol. 13(2):397-406 use the McDonald & Kreitman (supra) test as well as a method attributed to Nei and Gojobori, Nei and Jin, and Kumar, Tamura, and Nei; examining the average proportions of Pn, the replacement substitutions per replacement site, and Ps, the silent substitutions per silent site, to look for evidence of positive selection on binding genes in sea urchins to investigate whether they have rapidly evolved as a prelude to species formation. Goodwin et al. (1996) Mol. Biol. Evol. 13(2):346-358 uses similar methods to examine the evolution of a particular murine gene family and conclude that the methods provide important fundamental insights into how selection drives genetic divergence in an experimentally manipulatable system. Edwards et al. (1995) use degenerate primers to pull out MHC loci from various species of birds and an alligator species, which are then analyzed by the Nei and Gojobori methods (dN:dS ratios) to extend MHC studies to nonmammalian vertebrates. Whitfield et al. (1993) Nature 364:713-715 use KA/KS analysis to look for directional selection in the regions flanking a conserved region in the SRY gene (that determines male sex). They suggest that the rapid evolution of SRY could be a significant cause of reproductive isolation, leading to new species. Wettsetin et al. (1996) Mol. Biol. Evol. 13(1):56-66 apply the MEGA program of Kumar, Tamura and Nei and phylogenetic analysis to investigate the diversification of MHC class I genes in squirrels and related rodents. Parham and Ohta (1996) Science 272:67-74 state that a population biology approach, including tests for selection as well as for gene conversion and neutral drift are required to analyze the generation and maintenance of human MHC class I polymorphism. Hughes (1997) Mol. Biol. Evol. 14(1):1-5 compared over one hundred orthologous immunoglobulin C2 domains between human and rodent, using the method of Nei and Gojobori (dN:dS ratios) to test the hypothesis that proteins expressed in cells of the vertebrate immune system evolve unusually rapidly. Swanson and Vacquier (1998) Science 281:710-712 use dN:dS ratios to demonstrate concerted evolution between the lysin and the egg receptor for lysin and discuss the role of such concerted evolution in forming new species (speciation). Messier and Stewart (1997) Nature 385:151-154, used KA/KS to demonstrate positive selection in primate lysozymes.
The genetic changes associated with domestication have been most extensively investigated in maize (the preferred agricultural term for corn) (Dorweiler (1993) Science 262:232-235). For maize, (Zea mays ssp. mays), a small number of single-gene changes apparently accounts for all the differences between our present domesticated maize plant and its wild ancestor, teosinte (Zea mays ssp paruiglumis) (Dorweiler, 1993). QTL (quantitative trait locus) analysis has demonstrated (Doebley (1990) PNAS USA 87:9888-9892) that no more than fifteen genes control traits of interest in maize and explain the profound difference in morphology between maize and teosinte (Wang (1999) Nature 398:236-239).
Importantly, a similarly small number of genes may control traits of interest in other grass-derived crop plants, including rice, wheat, millet and sorghum (Paterson (1995) Science 269:1714-1718). In fact, for most of these relevant genes in maize, the homologous gene may control similar traits in other grass-derived crop plants (Paterson, 1995). Thus, identification of these genes in one grass-derived crop plant would facilitate identification of homologous genes in all of the others.
As can be seen from the papers cited above, analytical methods of molecular evolution to identify rapidly evolving genes (KA/KS-type methods) can be applied to achieve many different purposes, most commonly to confirm the existence of Darwinian molecular-level positive selection, but also to assess the frequency of Darwinian molecular-level positive selection, to elucidate mechanisms by which new species are formed, or to establish single or multiple origin for specific gene polymorphisms. What is clear is from the papers cited above and others in the literature is that none of the authors applied KA/KS-type methods to identify evolutionary changes in domesticated plants and animals brought about by artificial selective pressures. While Turcich et al. (1996) Sexual Plant Reproduction 9:65-74, describes the use of KS analysis on plant genes, it is believed that no one has used KA/KS type analysis as a systematic tool for identifying in domesticated plants and animals those genes that contain evolutionarily significant sequence changes that can be exploited in the development, maintenance or enhancement of desirable commercial or aesthetic traits.
The identification in domesticated species of genes that have evolved to confer unique, enhanced or altered functions compared to homologous ancestral genes could be used to develop agents to modulate these functions. The identification of the underlying domesticated species genes and the specific nucleotide changes that have evolved, and the further characterization of the physical and biochemical changes in the proteins encoded by these evolved genes, could provide valuable information on the mechanisms underlying the desired trait. This valuable information could be applied to developing agents that further enhance the function of the target proteins. Alternatively, further engineering of the responsible genes could modify or augment the desired trait. Additionally, the identified genes may be found to play a role in controlling traits of interest in other domesticated plants. A similar process can identify genes for traits of interest in domestic animals.
All references cited herein are hereby incorporated by reference in their entirety.